Infrared detectors are required to have a substantial sensitivity to infrared radiation, which is a simple objective to attain. However in some applications these detectors will also be exposed to severe flux of ionizing nuclear radiation (neutrons, gamma rays and beta rays, sometimes called "hard" radiation), which severely inhibit their performance to such an extent that their response is unreliable.
This situation has long been recognized, and efforts have been made to correct it. One straight-forward approach is to recognize that the infrared radiation, which is of critical importance to sense and to measure can be focused, while the ionizing radiation cannot be focused. It simply passes straight through the system, and can damage infrared sensors, the damage being roughly proportional to the amount of hard radiation that impinges on it.
It follows, and has long been recognized, that if the infrared radiation from some larger aperture is focused on a smaller area of sensor, then this smaller area receives all of the infrared radiation but this smaller area receives only a much smaller proportion of the ionizing radiation, much or most of which passes through the system and misses the sensor. Thus the reduction in area of the sensor gives up nothing as to sensitivity to infrared radiation, but because of its smaller area it significantly reduces its own exposure to the ionizing radiation.
This correct theory has been applied quite satisfactorilly, except that it has an undesirably high limit of resolution. This limit is known as diffraction-limited. It is an object of this invention significantly to overcome this quantum mechanical limit, and thereby enable the employment of smaller sensors. In fact, with this invention the required detector size can be reduced in linear extent by about one order of magnitude, and in area by about two orders of magnitude, thereby similarly decreasing the risk of disablement of the sensor. This invention can thereby provide greatly increased tolerance (hardening) to ionizing radiation.
A detector according to this invention utilizes an array of reduced-area infrared detectors in immersion contact with a refracting body that terminates in a plurality of Fresnel lenses which act as field lenses. One such lens and one such detector are respective to each other in the array. The refractive body is a continuous structure from said detectors to said lenses.
It is a feature of this invention that the body (of which the Fresnel lens are constituted) has a substantial index of refraction relative to infrared wavelengths.
By combining the use of Fresnel lenses and immersion optics of the class described herein, the diffraction limits can be greatly reduced.